The present invention relates to a variable-reluctance analog position transducer device and method. It is applicable in particular but not exclusively to devices suitable for measuring a variation of air gap between a ferromagnetic target and a transducer, the ferromagnetic target being displaced in linear or rotary manner opposite the transducer.
In general, the invention comprises a contactless rotary or linear transducer making it possible to obtain very linear induction variation, with maximum sensitivity. It can find a very particular application in measurement of the liquid level in a reservoir, the target being placed inside the liquid-containing reservoir and the magnet being situated on the outside, thus measuring the position of the target through the reservoir walls.
For better understanding of the invention and of the magnetic transducer devices, it is important hereinafter to understand the notion of “zero gauss”.
The measurement position delivering a signal equal to zero gauss corresponds effectively to the point of greatest stability and sensitivity. There is no “offset”, or in other words a significant difference relative to zero gauss, to be amplified; only the useful signal has to be processed, thus permitting a larger gain from the electronic viewpoint, leading to a more favorable signal-to-noise ratio. In addition, a temperature-compensating device is necessary to compensate for the reversible change of the magnetic properties of the magnet. The variation of transducer sensitivity as a function of temperature must be compensated for, but the compensation applied cannot be perfect and the influence of these compensation errors will be smaller the smaller the measured inductance is.
At present there exist rotary transducer devices making it possible to obtain an excellent linearity, that is, of ±0.5%, albeit over travels of reduced angular range.
For example, there is already known French Patent No. 2670286 filed in the name of the Applicant, which describes a position transducer provided with a stator defining an air gap, within which there is displaced a mobile magnet integral with a coupling means. The transducer is provided with a Hall-effect sensor for measuring the variation of induction in the air gap. The stator is composed of a first fixed part and a second part that is either fixed or mobile, the two parts defining between one another a main air gap in which there is displaced the part of the mobile element. The mobile element has at least two adjacent thin parts magnetized transversely in alternate sense, the magnetized parts being made of a material having a practically linear demagnetization characteristic and a reversible permeability close to that of air in the entire working range. The fixed part has at least two secondary air gaps substantially perpendicular to the main air gap in which the mobile element is displaced. The Hall-effect sensor is seated in one of the said secondary air gaps.
There is known European Patent No. 0665416, also filed in the name of the Applicant, which describes a magnetic position transducer of the type provided with a radially magnetized thin permanent magnet of tubular shape, integral with a coupling shaft, the said permanent magnet being rotationally mobile in a cylindrical main air gap defined by a yoke and a stator of soft magnetic material, the stator having a secondary air gap in which there is disposed a Hall-effect sensor, characterized in that the stator is disposed coaxially in the interior of the permanent magnet, and in that the yoke ensuring closure of the magnetic flux generated by the permanent magnet is formed by a tubular piece coaxial with the magnet and the rotor.
Transducer devices with digital and not analog output are also found on the market. Such transducers are generally used for detection, that is, the presence or absence of a toothed ferromagnetic target. The output signal of this type of transducer is not proportional to the linear or angular position of this ferromagnetic target, because it has only two states, usually designated 1 or 0, as a function of that which is close to the transducer. Relative to devices of this type, French Patent Nos. 2735222 and 2734913 disclose a proximity transducer comprising a Hall-effect ferromagnetic piece. French Patent No. 2724722, filed in the name of the Applicant, also describes a device that permits the detection, that is, only the presence or absence of a toothed ferromagnetic target.
In the prior art there is known U.S. Pat. No. 4,785,242, which describes a type of angular transducer having a rotor of variable section for ensuring a transfer function between the angular position and the output signal. On the other hand, it does not disclose the characteristic according to which the Hall-effect sensor is seated in a cavity of the permanent magnet.
German Patent Application 19503075 describes a mechanical arrangement for positioning the sensor. This patent application does not disclose anything at all about a variable-reluctance analog position transducer seeking to create zero gauss.
By virtue of the cavity created in the permanent magnet in the device according to the invention, some of the lines of force pass through the cavity, leading to a “negative” field, while the other lines of force pass outside the magnet, leading to a “positive” field, the zero gauss point being situated between these two fields. By virtue of the zero gauss, there are achieved:                temperature stability: 0 gauss=0 gauss at all temperatures,        better signal amplification, because there is no common mode to be amplified.        
The technical solutions disclosed in the foregoing patents may be satisfactory in their particular application, but the major drawback of these devices lies in the fact that the function of measurement of angular position by a variation of the magnetic induction has a maximum travel of only ±90° around the zero gauss position.
At present, therefore, there does not exist any analog and not digital magnetic position transducer capable of determining the position of a ferromagnetic target over a travel comprising one complete revolution, or in other words 360°.